A boring head of this type is known for example from U.S. Pat. No. 4,953,638. The boring head described there is fixed to a boring rod by means of which the boring head can be rotated and pressingly be driven forwardly. The boring head has an essentially wedge shape. On one of the wedge surfaces is fastened a boring blade, while a jet is arranged on the other wedge face, the stream direction of the jet being inclined at a small angle with respect to the forward drive direction, that is the boring head axis, so that the jet faces approximately in the forward direction.
During a straight running the boring head must be rotated. For a change in direction the boring head is brought to a position in which the boring blade carrying wedge surface faces oppositely to the desired deflection direction. Then the boring head is statically advanced by the boring rod. The boring head therefore moves in the desired deflection direction because of the flat side of the boring blade which serves as a control surface. The boring fluid flowing from the jet serves to support the boring effect, that is to cool the boring blade and to wash away the region of earth cut out by the boring blade.
With the rotation of the boring head the boring fluid loosens the earth lying in front of the boring head in a cone lying relatively closely around the boring axis. In this way the resistance of the earth in the forward drive direction is reduced while on the other hand the earth located around the boring head remains firm. Therefore it is difficult to actually deflect the boring head in the desired direction with a pregiven radius corresponding to the inclination of the control surface. Moreover the danger exists that the jets will plug up if the boring takes place for only a short time without boring fluid.